1. Field of the Invention
This invention relates to a tank for producing aluminum by electrolysis of alumina dissolved in molten cryolite according to the Hall-Heroult process.
2. Description of the Prior Art
In the best performing installations producing aluminum by the Hall-Heroult process, the consumption of electric energy is at least equal to 13,000 KWh per ton of metal, and often exceeds 14,000. In a modern tank operating under a potential difference of 4 volts, the voltage drop in the electrolyte represents about 1.5 volts; it is therefore responsible for more than a third of the total energy consumption. This is due to the necessity of maintaining a sufficient distance between the anode and the cathode liquid aluminum sheet (at least equal to 40 mm and, most often, on the order of 50 to 60 mm) to prevent the reoxidation of the aluminum entrained to the anode by the movements of the sheet of liquid metal due to the magnetic effects and facilitated by the nonwettability of the carbon cathode substrate by the liquid aluminum.
To reduce the interpolar distance, without causing the entrainment of the cathode aluminum toward the anode, it has been proposed to use cathodes with a base of electroconductive refractories, such as titanium boride TiB.sub.2 which is completely wetted by the liquid aluminum and undergoes virtually no attack by this metal at the electrolysis temperature. These cathodes have been described, in particular, in the British Pat. Nos. 784,695, 784,696, 784,697 of BRITISH ALUMINUM CO., and in the article by K. B. BILLEHAUG and H. A. OYE in "ALUMINUM", October 1980, pages 642-648 and November 1980, pages 713 to 718.
One of the major problems that these titanium boride cathodes pose is their gradual going into solution in the liquid aluminum, a slow but not inconsiderable phenomenon which requires the periodic replacement of the elements used and involves total stopping and disassembly of the tank.